Environmental Problems, Their Causes, and Sustainability 1

obstacles. The warmer water and other conditions inthis zone support more producers (phytoplankton)and a variety of cool-water and warm-water fishspecies (such as black bass) with slightly lower oxygenrequirements.In the floodplain zone (see Figure 7-18, bottom),streams join into wider and deeper rivers that meanderacross broad, flat valleys. Water in this zone usuallyhas higher temperatures and less dissolvedoxygen than water in the first two zones. These slowmovingrivers sometimes support fairly large populationsof producers such as algae and cyanobacteriaand rooted aquatic plants along the shores. Because ofincreased erosion and runoff over a larger area, waterin this zone often is muddy and contains high concentrationsof suspended particulate matter (silt). Themain channels of these slow-moving, wide, andmurky rivers support distinctive varieties of fish (carpand catfish), whereas their backwaters support speciessimilar to those present in lakes. They are nice placesto fish. At its mouth, a river may divide into manychannels as it flows through coastal wetlands and estuaries,where the river water mixes with ocean water(Figure 7-7).As streams flow downhill, they become powerfulshapers of land. Over millions of years the friction ofmoving water levels mountains and cuts deep canyons,and the rock and soil the water removes are depositedas sediment in low-lying areas.Streams are fairly open ecosystems that receivemany of their nutrients from bordering land ecosystems.We have established farmlands and constructeddams, power plants that need cooling water, sewagetreatment plants, cities, recreation areas, and shippingterminals in the watersheds along the shores of riversand streams, especially in their transition and floodplainzones. This greatly increases the flow of plant nutrients,sediment, and pollutants into these ecosystems.To protect a stream or river system from excessive inputsof nutrients and pollutants, we must protect theland around it.What Are Freshwater Inland Wetlands?Natural SpongesInland wetlands absorb and store excess waterfrom storms and provide a variety of wildlifehabitats.Inland wetlands are lands covered with fresh waterall or part of the time (excluding lakes, reservoirs, andstreams) and located away from coastal areas. Wetlandsinclude marshes (dominated by grasses andreeds with a few trees), swamps (dominated by treesand shrubs), and prairie potholes (depressions carvedout by glaciers). Other examples are floodplains (whichreceive excess water during heavy rains and floods)and the wet arctic tundra in summer. Some wetlandsare huge and some are small.Some wetlands are covered with water yearround.Others, called seasonal wetlands, usually are underwateror soggy for only a short time each year.They include prairie potholes, floodplain wetlands,and bottomland hardwood swamps. Some stay dryfor years before being covered with water again. Insuch cases, scientists must use the composition of thesoil or the presence of certain plants (such as cattails,bulrushes, or red maples) to determine that a particulararea is really a wetland. Inland wetlands provide anumber of important and free ecological and economicservices such as filtering toxic wastes and pollutants,absorbing and storing excess water fromstorms, and providing habitats for a variety ofspecies.What Are the Impacts of Human Activitieson Freshwater Systems? Using and AbusingRivers and WetlandsWe have built dams, levees, and dikes that reducethe flow of water and alter wildlife habitats in rivers;established nearby cities and farmlands that pollutestreams and rivers; and filled in inland wetlands togrow food and build cities.Human activities have four major impacts on freshwatersystems. First, dams, diversions, or canals fragmentalmost 60% of the world’s 237 large rivers. This altersand destroys wildlife habitats along rivers and incoastal deltas and estuaries by reducing water flow.Second, flood control levees and dikes built alongrivers alter and destroy aquatic habitats. Third, citiesand farmlands add pollutants and excess plant nutrientsto nearby streams and rivers.Fourth, many inland wetlands have been drainedor filled to grow crops or have been covered with concrete,asphalt, and buildings. In the United States,more than half of the inland wetlands estimated tohave existed in the lower 48 states during the 1600s nolonger exist. The total area of these wetland losses isgreater than the size of California (which has lost 90%of its wetlands), Oregon, and Nevada combined. Suchloss of important natural capital has been an importantfactor in increased flood and drought damage in theUnited States. Many other countries have sufferedsimilar losses. For example, 80% of all wetlands inGermany and France have been destroyed.In this chapter we have seen that human activitiesare severely stressing and overloading many of theworld’s aquatic systems. Good news. Research showswhen such activities are reduced most of these systemscan recover fairly quickly as long as aquatic life zonesare not overfished or overloaded with pollutants andexcessive nutrients.http://biology.brookscole.com/miller14141